Conventionally, a seat frame of pan shaped structure is made integrally of plastic resin material, or made of molded steel plate. When the seat frame is produced by press-molding, side frames and the center panel are formed to have uniform thickness since they are integrally molded.
However, the side frames of the pan structure frame should be more stronger and should be thicker than the center panel since a device for adjusting seat sliding, a device for changing the seat angle, and so on, are fixed to the frames, and since load from a back seat is applied to the frames by way of the device for changing the seat angle.
In order to produce thick side frames, however, the whole seat frame itself including its center panel, is inevitably made of thick steel plate, which results in increasing the total weight. Therefore, the whole seat frame has been made of relatively thin steel plate so as reduce the total weight, and reinforcing plates have been fixed to the side frames to increase their strength.
According to these conventinal processes, however, since side frames are formed complexed structure, a number of production steps are necessary for producing the side frames. In addition, the setting position to stop the device for sliding the seat, or that for adjusting seat angle, is limited only to certain area. This results in inconveniences for designing the seat frame and in requiring an insufficient number of steps.